The present invention relates to the LTE (Long Term Evolution) wireless communication system defined by the 3GPP standardization committee and included in the so called Release8 specification. Long Term Evolution (LTE) corresponds to the more recent development in wireless cellular communications, following the High Speed Downlink Packet Access HSDPA and High Speed Uplink Packet Access (HSUPA).
The mechanism of cell detection in LTE is based on the use of the so-called Primary Synchronization Signal (PSS) which is one of the two synchronization signals—namely the primary synchronization signal (PSS) and the secondary synchronization signal (SSS). The identification of the indices of the PSS and SSS sequences transmitted by the base station allows achieving the physical layer Cell ID detection.
The PSS signal is a signal which is based on the use of ZC sequences showing interesting correlation properties, and is periodically transmitted each 5 ms (i.e. half a radio frame) by a cell or a base station over six resource blocks comprising 72 central sub-carriers independently of the transmission bandwidth.
It is received by the User Equipment (UE) for detecting the initial timing and might be used for computing the strength of the signal. One may refer to document 3GPP TS 36.211, “Physical Channels and Modulation”, V8.6.0, 17 Mar. 2009.
As known by a skilled man, the aim of this synchronization signal is to ensure coarse time and frequency synchronization of the User-Equipment (UE), allowing the latter to obtain the slot timing and to acquire the carrier frequency and a part of the cell-identity within a physical-layer cell-identity group.
Once the PSS signal has been detected, the UE proceeds with additional processing including the detection of the SSS allowing completion of the synchronization process and the detection of the start of the frame as well as the other part of the cell ID i.e. the physical-layer cell-identity group.
A proper detection of the PSS shows to be a critical operation in order to avoid unnecessary waste of time, power and digital processing resources to be consumed in the case of false detections.
Typical PSS detection procedure consists of the cascade of:
1) Correlation with PSS time domain sequences and accumulation: This step comprises the computation of the correlation of the received signal with respect to known PSS time sequences for all timing samples within a LTE radio frame and eventually accumulation over a set of half radio frames, i.e. duration of 5 ms, (or a integer multiples of this duration) to improve the sensitivity.
2) Constant-False-Alarm-Rate (CFAR) selection: consisting in applying a threshold to the correlation values. The threshold is determined in function of the estimate of the noise variance and of a target False-Alarm-Rate (FAR). Such method has the drawback of showing a high CFAR for a given PSS detection probability.
As known by the skilled man, there is a clear trade-off between the noise selectivity (low FAR) and the PSS signal detection probability. A higher threshold may reduce the occurrence of false alarms due to noise peaks, but has unfortunately the drawback of reducing probability of detection of the true PSS peaks.
The existence of these un-wanted detections due to noise results in a large amount of peaks that need to be further processed by the common LTE cell-search and synchronization physical procedure consisting in Secondary-Synchronization Signal (SSS) detection, to acquire the physical-layer cell-identity group and the Radio frame boundary. It is clear that false-alarms constitute the cause of energy wastage especially for idle-mode UE operations, thus resulting in a significant shortening of the battery life.
Therefore, there is a clear need for an improved technique for reducing the overall quantity of false alarms while maintaining at a reasonable and acceptable level the probability of detection of true PSS peaks.